Step 1 Make the Cupcakes: Preheat oven to 350 degrees F. Line two standard muffin tins with liners.
Step 2 Place the Guinness and butter in a medium saucepan and bring to a simmer over medium heat. Add the cocoa powder and whisk until the mixture is smooth. Remove from the heat and cool slightly.
Step 3 In a medium bowl, whisk together the flour, sugar, baking soda, and salt; set aside.
Step 4 Using an electric mixer, beat the eggs and sour cream on medium speed until combined. Add the Guinness-chocolate mixture to the egg mixture and beat just to combine. Reduce the speed to low, add the flour mixture and beat just until it starts to come together, about 30 seconds. Using a rubber spatula, fold the batter until completely combined. Divide the batter among the cupcake liners. Bake until a thin knife inserted into the center comes out clean, about 17 minutes. Cool for 5 minutes in the pan, then remove the cupcakes to a wire rack to cool completely.
Step 5 Make the Whiskey Ganache Filling: Place the chocolate in a heatproof bowl. Place the heavy cream in a small saucepan and bring to simmer over medium heat. Immediately pour it over the chocolate, then let it sit for two minutes. Using a rubber spatula, gently stir the mixture from the center outward until smooth. Add the butter and whiskey and stir until combined. Let the ganache cool until thick but still soft enough to be piped, about 30 minutes. (If it becomes too stiff, simply give it a good whisk and it will loosen up.)
Step 6 Fill the Cupcakes: Using a paring knife, cut the centers out of the cooled cupcakes, going about two-thirds of the way down. Using a cookie scoop or spoon, divide the prepared ganache between the centers of the cupcakes.
Step 7 Make the Baileys Frosting: Using the whisk attachment of a stand mixer, whip the butter on medium-high speed for 5 minutes, scraping the sides of the bowl occasionally. Reduce the speed to medium-low and gradually add the powdered sugar until all of it is incorporated. Add the Baileys, increase the speed to medium-high and whip for another 2 to 3 minutes, until it is light and fluffy.
Step 8 Using your favorite decorating tip, or an offset spatula, frost the cupcakes and decorate with sprinkles, if desired. Store the cupcakes in an airtight container at room temperature for up to 4 days.
